Theory of Graded-Bandgap Thin-Film Solar Cells

Enhanced efficiency of graded-bandgap thin-film solar cells due to concentrated sunlight

“…1(a). Detailed geometrical, optical, and electronic descriptions of these solar cells are available elsewhere [12,13,15,16,24]. A brief review is as follows.…”

“…Three different types of thin-film solar cells-CIGS, CZTSSe, and AlGaAs-were considered, as these are widely investigated thin-film solar cells with the absorber layer made of a compound semiconductor. A coupled optoelectronic model [22][23][24] and the differential evolution algorithm (DEA) [25] were used together to maximize the efficiency η against the bandgap-grading parameters [24] and the sunlight-concentration factor c sun . The bandgap of the absorber layer in each thinfilm solar cell was considered to be nonlinearly graded along the thickness direction.…”

“…The theoretical results presented are based on a two-step optoelectronic model [22][23][24]. In the optical step of this model, the transfer-matrix method [26,27] is used for CIGS and CZTSSe solar cells to determine the electron-hole-pair (EHP) generation rate G inside the solar cell [12,13], assuming normal illumination by unpolarized polychromatic light endowed with the AM1.5G solar spectrum [28].…”

“…In the optical step of this model, the transfer-matrix method [26,27] is used for CIGS and CZTSSe solar cells to determine the electron-hole-pair (EHP) generation rate G inside the solar cell [12,13], assuming normal illumination by unpolarized polychromatic light endowed with the AM1.5G solar spectrum [28]. The rigorous coupled-wave approach (RCWA) [24,27] is used for the AlGaAs thinfilm solar cells as it involves localized Pd-Ge-Au ohmic back-contacts embedded periodically in an Ag backreflector (grating) [18,27,29]. The transfer-matrix method is an efficient technique to compute the reflectances, transmittances, and absorptances of a multilayered structure with a plane-stratified morphology, as shown in Fig.…”

“…In the electrical step of the model, G appears as a forcing function in the system of onedimensional (1D) drift-diffusion equations applied to the semiconductor portion of the solar cell [30]. These equations are solved using a hybridizable discontinuous Galerkin scheme [31][32][33] to determine the device current density J dev and the electrical power density P as functions of the external bias voltage V ext [24]. The coupled optoelectronic model has been validated against several experimental results on solar cells with a homogeneous-bandgap absorber layer [12,13,15,16,18].…”